In april Im going to Athens with school, and Im going to bring my Panasonic FZ28 with me instead of the 350D. Why? Because of its great IQ and portability, and I dont want to lose my 350D...
I got an issue though. The polarfilter of my FZ28 gives really bad vignetting, there arent any corners @27mm, it just looks like a fisheye effect on a FF. I dont know how bright the sky will be in april, but do I have to use the polarfilter? If Ill use it, I have to crop really big parts of the images, and I think Ill need the 27mm coverage.
What should I do? Polarfilter and much cropping, or no polarfilter? I really love those blue skies though...
I can of course try this filter in hope to be not the same as the one I (my father) own : http://everbuying.com/46mm-Camera-Filter-CPL-p-19678.html

Also, a bit offtopic, what should I bring with me on the trip? Do you know important things I have to bring with me (standard things I bring of course with me...) and do you know some nice places in Athens? Any Greek people here?

Personally, I believe "you get what you pay for" that being said I would not buy a $6 CPL filter. For my filters I only go with Hoya, I use them on my digital and film cameras and have been very happy with them, I am sure others will attest to Hoya's quality. A link is below to a Hoya 46mm CPL.

Seeing that you are in Holland, I know you don't want to order from that site because shipping is outrageous to Europe (I was going to order something to Rome where I am living and shipping was $50) Try finding a local camera shop where you can try out different filters to see if you get the vignetting problem.